Reliable fire detection is critical for many aircraft. Common thermal fire detector types include point thermocouple, point thermistor, continuous thermocouple, continuous thermistor, resistance wire, and pneumatic tube.
Some pneumatic tube detector systems include a titanium or vanadium wire inserted into a capillary sensor tube. The wire is exposed to and absorbs high temperature energy and pressurized hydrogen gas and subsequently stores the gas as the wire cools to form a hydrogen saturated wire. This saturated wire is inserted into a sensor tube, which is pressurized with an inert gas, and sealed at both ends to form a pressure vessel, which can be used as a pneumatic detector. One of the ends of the pressure vessel is incorporated into a housing that includes a plenum, where alarm and integrity switches are located.
When the sensor tube portion of the pneumatic detector is exposed to high temperature, the pressure is increased inside the tube as the inert gas expands in accordance to physical gas laws. Such pneumatic fire detectors may include diaphragms that are pre-formed prior to assembly. The detectors may also include part of the gas seal for the device. The diaphragms may be pre-formed to operatively position the diaphragm in various positions such as, for example: (a) an open switch (alarm switch) condition requiring the background pressure to increase to create a closed or alarm condition; or (b) a maintained closed switch (integrity switch) condition with the background pressure.
For an alarm switch configuration, the diaphragm may be deformed so it is responsive to a predetermined background pressure to further deform sufficiently outward and create a closed switch. The diaphragm may also be deformed such that a portion of the interior side of the disc forms part of the pressure seal for the plenum. With this configuration, in the event of an overheat or fire condition, pressure in the sensor tube and plenum will rise. If a predetermined high temperature condition is reached, the pressure within the plenum will increase to such an extent that the diaphragm will be deformed outward and into electrical contact and create a closed switch.
For an integrity switch configuration, the diaphragm may be deformed so that the diaphragm responds to a predetermined drop in background pressure and deforms sufficiently inward to lose electrical contact with a switch. The diaphragm may also be deformed such that a portion of the interior side of the diaphragm forms part of the pressure seal for the plenum. With this configuration, the integrity switch opens if a loss of pressure occurs in the sensor tube or plenum. If a predetermined pressure loss occurs, the pressure within the plenum will decrease to such an extent that the diaphragm will lose electrical contact and create an open switch.
However, such pneumatic detectors may have complex designs, numerous parts, and may be hard to manufacture. Accordingly, it is desirable to provide a pneumatic detector assembly having a less complex design, fewer parts, reduced amount of brazing or welding of parts, overall ease of manufacture, and reduced tests to ensure hermeticity of the assembly.